Variables: HRT, Heart rate; SDNN, Standard deviation of normal to normal interval; RMSSD, the square root of the mean squared differences of successive NN intervals; PSI, Physical stress index; ApEn, Approximate Entropy; SRD, Successive R-R interval difference; TSRD, Total SRD;TP, Total power; VLF, Very low frequency; LF, Low frequency; HF, High frequency; nLF, normalized LF; nHF, normalized HF.